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In this paper, the interaction of 10-TW laser pulses, focused at moderately relativistic intensity, with a supersonic he-lium gas-jet has been investigated by varying gas density and jet nozzle. We have successfully tested several advanced diagnostic devices to characterize the plasma and the accelerated electron bunches. Plasma densities have been measured by means of a fem-tosecond high-resolution interferometer, while the electron beams were analyzed with a stack of radiochromic films, a beam-profile monitor, a magnetic spectrometer, and a nuclear activation setup based on gamma-ray generation via electron bremsstrahlung. We present the results as well as the basic features and relevant details of such diagnostics whose performances can fit a large class of experiments.

X-ray spectroscopic diagnostics of laser-cluster interaction at the stage of nonadiabatic scattering of clusters and formation of a spatially uniform plasma channel has been performed. The experimental investigations have been carried out on a Ti:Sa laser setup with a pulse duration of about 65 fs and an energy up to 600 mJ. It has been shown that, within 10 ps from the beginning of a laser femtosecond pulse, the laser-cluster interaction forms a uniform plasma channel with a length of 0.4 to 1 mm with the parameters N e ∼ 1019−1020 cm−3 and T e ∼ 100 eV.

In this work, we demonstrate that the contrast of high intensity laser pulses is a key issue for many problems dealing with high-field interaction with solid-density matter. As an example, we will discuss the interaction of an Ultra-High-Contrast (UHC), Ultra-Intense, 65 fs laser pulse with an overdense plasma through the proton acceleration from ultra-thin foils (from 0.08 μm to 105 μm). The symmetric feature of the ion beams emitted from both faces of the target will be demonstrated as well as the dominant role of the p component of the electric field rather than the ponderomotive force. Simulations performed with a 2D particle-in-cell code are in close agreement with all experimental data. UHC experiments prove to be a good as benchmarks for theories and models.

We investigate the interaction of a high-intensity (~5.1018 W/cm2) and short (~65 fs) laser pulse with thin foils (from 0.08 to 105 mum) in a regime of ultrahigh contrast (> 1010). This paper shows that for thicknesses less than about 10 mum, proton acceleration from both sides of the target presents quite symmetric features. Proton bunches emitted from each side show similar maximum energies and spatial characteristics. Moreover, we show that for ultrahigh-contrast pulses, the efficient acceleration mechanism is related to the Brunei effect and not to the ponderomotive force. Simulations performed with a 2-D particle-in-cell code are in close agreement with all experimental data.

International audience; A new method to improve the temporal contrast of an ultra-short laser pulse has been developed. This technique uses a Sagnac interferometer in combination with a non-linear medium. A phase shift induced in the ultra-short pulse by non-linear effect separates it from the amplified spontaneous emission background. Experiments with a 1 kHz, 55 fs, 1 mJ Ti:Sa laser were performed. The temporal contrast measurements have shown a contrast improvement of four orders of magnitude.

We demonstrate the influence of the prepulses and ASE of ultrashort pulses interacting with a solid target by addressing the direct comparison of the harmonic spectra generated by reflection onto a solid target with and without the introduction of a plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free from any plasma expansion.

A portable (200 � 100 � 100 mm 3 ), high-luminosity, spherically bent crystal spectrometer was designed to measure very low emissivity X-ray spectra of different elements with spatial resolution in a wide spectral range (1.2� 19.6 ( A). A large (50 � 15 mm 2 ) open aperture mica spherically bent crystal with R ¼ 150 mm was used as dispersive and focusing element. This spectrometer was associated with a large sensitive area (f ¼ 40 mm) micro-channel plates assembly. This apparatus provides simultaneously high spectral (l=dlB1800) and spatial (100–200mm) resolutions. Its large tunability allowed, without any adjustment of the spectrometer set-up, to record spectra in the 1.38–17.5 ( A wavelength range. We used the X-ray emission of femtosecond laser-produced plasmas from different materials ((CF2)n, CaF2, Cu, Al) to test the spectrometer. Thanks to the high sensitivity (high collection efficiency) of the system, high quality space-resolved X-ray spectra of Fluorine and Aluminum plasmas were obtained on a single laser shot. r 2001 Elsevier Science B.V. All rights reserved.

International audience; A general approach for optically controlled spatial structuring of overdense plasmas generated at the surface of initially plain solid targets is presented. We demonstrate it experimentally by creating sinusoidal plasma gratings of adjustable spatial periodicity and depth, and study the interaction of these transient structures with an ultraintense laser pulse to establish their usability at relativistically high intensities. We then show how these gratings can be used as a “spatial ruler” to determine the source size of the high-order harmonic beams produced at the surface of an overdense plasma. These results open new directions both for the metrology of laser-plasma interactions and the emerging field of ultrahigh intensity plasmonics.

Atomic argon clusters were created using various types of nozzles and gas jet backing pressures. High temperature plasmas were created by irradiating the clusters with high intensity 35 fs laser pulses. The data obtained from these experiments were compared with each other as well as an experiment using a longer pulse ( 60 fs ) higher flux density laser. Detailed spectroscopic analysis of high resolution X-ray data near the Heα,Heβ, and Lyα lines of Ar was consistent with a two-temperature collisional-radiative model incorporating the effects of highly energetic electrons. Each variation has an effect on the rate of cluster expansion; therefore, results of the analysis provide useful insight into the behavior of such plasmas over short time scales. It was demonstrated for the first time, that by using sufficiently large clusters, hydrogen-like ions of Ar with an ionization potential of approximately 4120 eV could be obtained with a 35 fs laser pulse with a relatively low flux density of 1×10 17 W/cm 2 .

The generation of fast particles (both ions and electrons) in a femtosecond laser-produced plasma has been studied experimentally and the possibility of applying particle-in-cell (PIC) codes to the description of the experimental results has been investigated. The energy distribution function of fast ions has been measured directly by means of X-ray spectroscopy. It is shown that this diagnostic can also be used as an indirect method to measure fast electrons inside the plasma. The hot electron distribution in the 0:1–2 MeV energy range was measured directly using a standard electron magnetic spectrometer. The comparison of the experimental results with the numerical simulations show: (1) the electromagnetic PIC-FC code can be successfully applied for simulating plasmas created by the interaction of femtosecond-laser pulses with solid targets and (2) the existence of a transient population of hot electrons with a non-Maxwellian distribution typical of interaction experiments with short-pulse lasers. ? 2001 Elsevier Science Ltd. All rights reserved.

A study on the interaction of a 10 TW, 60 fs, Ti–Sapphire laser with a high-density long-scale pulsed nitrogen gas jet is reported. Experimental data on the laser propagation are analyzed with the help of a ray-tracing model. The plasma dynamics is investigated by means of time-resolved shadowgraphy and time-integrated high-resolution x-ray spectroscopy. Shadowgrams show that the plasma does not expand during the first 55 ps, while x-ray spectra exhibit an unusual continuum-like structure attributed to hollow atoms produced by charge exchange process between bare nuclei expelled from the plasma and molecules of the surrounding gas. The interpretation of the results is supported by particle-in-cell simulations. The question of x-ray lasing is also examined using a hydrodynamic code to simulate the long lasting regime of recombination.

We recall the main characteristics of high harmonic generation (HHG) by intense laser pulses in gas, which proceed from the fact that HHG is a laser-driven non linear process. We particularly outline the properties of intrinsic, spatial and temporal coherence which are investigated experimentally. We show that they can be controlled through various phase-matching parameters, on account of the particular form of the atomic dipole phase involved in HHG. Finally, we present two feasibility experiments demonstrating that this XUV source can be effectively used in time-resolved interferometry measurements, in particular in the diagnostic of laser-produced ultra-short dense plasmas.

High order harmonic generation in gases is now a widely studied non linear process. The research in this domain shows two main directions. On the one hand, researchers are working on the different fundamental aspects involved in high order generation. On the other hand, they have developed several applications of this unique XUV source. High order harmonics (HOH) present a good beam quality, which allows to focus the harmonic radiation to very tight spots1–3, a high brightness and a high repetition rate (up to 1 kHz)4 –6. Since high order harmonic generation is a field driven process, the properties of the laser that is used to generate the harmonics are partially transferred to the XUV radiation. As a result, the harmonic pulse presents a short duration, equal to or shorter than the generating laser pulse duration 7 ,8 depending on the harmonic order. The harmonic radiation is also naturally synchronized with the fundamental driving IR laser. These two last properties are very important for all experiments of the pump probe type. Up to now high order harmonics have already been used in atomic and molecular spectroscopy9,10. This type of experiments can also be applied in solid state physics for instance to probe the relaxation time of electrons that have been excited in the conduction band of a dielectric material11. Another important property of the harmonic source is the good temporal12 and spatial13–15 coherence that can be obtained in some generation conditions. Moreover, a very unique property in this XUV range is the possibility to produce two mutually coherent sources either separated in space or in time. In the following, we discuss the generation of these phase-locked harmonic sources and their applications to plasma diagnostic.

The interaction of a 60-fs laser pulse with argon clusters was experimentally studied. It is shown that cluster heating by an intense femtosecond pulse preceded by a picosecond prepulse can produce plasma whose ionization state is determined by the prepulse properties, while the mean energy of hot electrons is determined by the main pulse intensity. A simple model of cluster plasma evolution is suggested, allowing an adequate description of its X-ray emission.

An x-ray laser scheme based on the recombination of a fully stripped nitrogen plasma is presented. Plasma is assumed to be created by the optical-field ionization of a nitrogen gas jet of 10(19) cm-3 atomic density by an ultrashort (60 fs), high-intensity (3 x 10(19) W/cm2) Ti:sapphire laser. Results of two-dimensional particle-in-cell simulations, modeling laser-plasma interaction, parametric heating, and ponderomotive effects are presented. Hydrodynamic and kinetics calculations are performed and predict important local gain for H-like nitrogen transitions at 25 and 134 A, following fast collisional recombination for specific plasma conditions.

New types of space resolved X-ray spectra produced in light matter experiments with high intensity lasers have been investigated experimentally and theoretically. This type of spectra is characterised by the disappearance of distinct resonance line emission and the appearance of very broad emission structures due to the dielectronic satellite transitions associated to the resonance lines. Atomic data calculations have shown, that rather exotic states with K-shell vacancies are involved. For quantitative spectra interpretation we developed a model for dielectronic satellite accumulation (DSA-model) in cold dense optically thick plasmas which are tested by rigorous comparison with space resolved spectra from ns-lasers. In experiments with laser intensities up to 10 19 W/cm 2 focused into nitrogen gas targets, hollow ion configurations are observed by means of soft X-ray spectroscopy. It is shown that transitions in hollow ions can be used for plasma diagnostic. The determination of the electron temperature in the long lasting recombining regime is demonstrated. In Light-matter interaction experiments with extremely high contrast (up to 10 10 ) short pulse (400 fs) lasers electron densities of n e ≈3×10 23 cm −3 at temperatures between kT e =200–300 eV have been determined by means of spectral simulations developed previously for ns-laser produced plasmas. Expansion velocities are determined analysing asymmetric optically thick line emission. Further, the results are checked by observing the spectral windows involving the region about the He α -line and the region from the He β -line to the He-like continuum. Finally, plasmas of solid density are characteristic in experiments with heavy ion beams heating massive targets. We report the first spectroscopic investigations in plasmas of this type with results on solid neon heated by Ar-ions. A spectroscopic method for the determination of the electron temperature in extreme optically thick plasmas is developed.

We demonstrate that frequency-domain interferometry can be performed in the extreme ultraviolet range using high-order harmonics. We first show that two phase-locked harmonic sources delayed in time can be generated in the same medium despite ionization. This gives insight into the dynamics of the generation and ionization processes. We then apply the technique to the study of the temporal evolution of an ultrashort laser-produced plasma at the femtosecond time scale.

We present the characteristics of a high-density pulsed gas jet developed to study the interaction of a short-pulse, high-intensity laser with a large scale underdense plasma. The jet was characterized by laser interferometry. Measurements of density profile were performed for a 5 mm diam cylindrical nozzle and for a 20×1 mm slit. In this latter case, a systematic study of the output density as a function of the backing pressure and of the valve opening duration is presented. At 1 mm from its outlet, the cylindrical nozzle provides densities up to 6.9×1019 cm−3 for an 8 bar backing pressure with a 3 mm full width at half maximum Gaussian-shaped profile. Under the same conditions, the slit produces a 20 mm long flat-top profile with a maximum density of 2.1×1019 cm−3.

L'idee de ces journees est nee du besoin de coordonner les efforts des equipes travaillant sur les nouvelles sources X et XUV intenses et breves. Ces sources, qui arrivent maintenant a maturite, commencent a offrir des possibilites nouvelles d'applications pour de nombreux domaines de recherche. Il a paru necessaire de faire le point des resultats obtenus recemment a la fois sur les sources et sur leurs applications, afin de lancer une prospective et de rechercher les moyens d'organiser cette communaute scientifique, harmoniser les programmes et rationaliser l'utilisation des moyens.Une question particulierement importante et difficile a ete revelee avec force au cours de ces journees. Il s'agit des besoins en optique multicouche, qui sont presque exclusivement satisfaits par des laboratoires de recherche qui ne peuvent consacrer qu'une part marginale de leur activite a cette production. En revanche, le marche est actuellement trop faible pour etre pris en charge par les entreprises a un prix compatibles avec les moyens de la recherche publique.Le recueil des exposes presentes au cours de ces journees sera publie prochainement par l'Universite Paris-Sud.

For the first time registration of high-resolution soft x-ray emission and atomic data calculations of hollow-atom dielectronic satellite spectra of highly charged nitrogen have been performed. Double-electron charge-exchange processes from excited states are proposed for the formation of autoionizing levels in high-intensity laser-produced plasmas, when field-ionized ions penetrate into the residual gas. Good agreement is found between theory and experiment. Plasma spectroscopy with hollow ions is proposed and a temperature diagnostic for laser-produced plasmas in the long-lasting recombining regime is developed.

A hydrodynamic model of the interaction of a short, intense laser pulse with moderate-density gases is constructed. The model systematically takes into account both tunneling-ionization processes and the relativistic motion of the electrons that are produced by the ionization of the gas in the relativistically strong field of the pulse. X-ray spectroscopy data agree well with the proposed theory.

We use a time resolved interferometric method to study the kinetics of carriers trapping and defects formation following electronic excitation by high intensity ultrashort laser pulses in alkali halides. A systematic study of initial excitation density dependence of trapping kinetics, as well as the comparison of pure and doped samples, allows to investigate in detail the mechanism of energy relaxation and carriers localization accompanying the lattice rearrangement. The experimental results concern samples of pure and NO−2 doped KBr, and of pure NaCl. We observe faster trapping in KBrNO−2 than in pure KBr. In NaCl, the trapping rate increases with excitation density. These results show that in both materials the hole trapping is the primary process towards radiation induced defects. Furthermore, in KBr our results give strong evidence for the existence of an intermediate state with a short lifetime in the relaxation pathway between the initial electron–hole pairs and the final F–H pairs. A simple model describing the modification of the refractive index and rate equations governing free and trapped electron and holes populations are used to quantitatively interpret the experimental results.

International audience; We address the on target focal spot spatio-temporal features of an ultrashort, 100 TW class laser chain by using spectrally resolved imaging diagnostics. The observed spatio-spectral images, which we call rotating imaging spectrographs, are obtained single shot to reveal the essential information about the spatio-temporal couplings. We observe nontrivial effects in the focal plane due to compressor defects which significantly affect the maximum on target intensity. This diagnostic might become an essential tool for improving compressor alignment in many upcoming multi-petawatt short pulse laser facilities.

A novel one-colour pump - probe scheme with 130 fs pulses is used to study the fragments arising from the multi-electron dissociative ionization of molecular chlorine. This approach allows us to demonstrate the coexistence of nascent fragments and those fragments which are further ionized by the probe pulse. A simple model, based on tunnel ionization theory, accounts well for the observed population transfer of the ionic fragments. In contrast, only nascent fragments are observed for intense 2 ps pulses. Our results show that the absence of post-dissociation ionization in most experiments is not due to the strong decrease of the ionization rates beyond the critical distance. A possible explanation is laser-field induced screening of the nuclei by electrons leading to a `retarded' Coulomb repulsion. The multiple ionization of the molecule followed by dissociation would then occur later in the pulse where the intensity increase is not sufficient to induce post-dissociation ionization.

Two-color two-photon femtosecond ionization experiments have been performed on NaI. The wave packet evolution of the A excited state has been followed by detecting photoions and photoelectrons. The results indicate that the Na+ ions are formed when the wave packet is located at the outer turning point of the excited state. Surprisingly, the NaI+ ions are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ionizing around the outer turning point, in agreement with the NaI+ formation. The absence of signal corresponding to ionization from the covalent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent region of the A state (where the molecule can be considered as a van der Waals complex) than in the ionic Na+···I- part of the A state potential (where the interatomic distance is such that the ionization process may be considered as a photodetachment of...

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, has been experimentally investigated. Ultrahigh contrast ($\sim 10^{12}$) pulses allowed to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultra-high intensity $>10^{19} \mbox{W/cm}^{2}$. A maximum increase by a factor of 2.5 of the cut-off energy of protons produced by Target Normal Sheath Acceleration has been observed with respect to plane targets, around the incidence angle expected for resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance., 5 pages, 5 figures, 2nd version implements final corrections

Lasers delivering high intensity ultrashort pulses are adequate tools to study the dynamics of irradiation induced modifications in wide band gap materials. We present the results of two complementary time-resolved pump-probe optical experiments. We use a technique of interferometry in the frequency domain to probe the instantaneous modification of the refractive index of the material induced by a pump pulse. The presence of free carriers in the conduction band induces a negative phase shift of the probe pulse which is proportional to the excitation density. Comparison between different oxides reveals an extremely contrasted behaviour: the photoexcited carriers are trapped in 150 fs in SiO 2 , while they remain quasi-free for more than 50 ps in MgO and Al 2 O 3 . These lifetimes are closely connected to the formation of radiation induced defects. In particular, using a conventional time-resolved absorption technique in the UV (5.2–5.6 eV), we have studied the intrinsic defects formation kinetics in SiO 2 . Our results show that the formation of self-trapped excitons (STE) is occuring in the ultrashort time of 150 fs, in agreement with the free carrier lifetime measured in the above experiment. At low temperature, the STE recombine radiatively, while at temperature above the luminescence quenching temperature, a small proportion of the STE is converted into permanent defects (O vacancies).

We report the investigation of the onset of an absorption band in the UV in crystalline quartz () induced by an intense femtosecond laser pulse. Using a conventional pump - probe technique, we have measured the absorptions at 219 nm (5.66 eV) and at 240 nm (5.16 eV) as functions of time, at two different temperatures (10 and 300 K). The rise time of the absorption is measured to be 150 . It is independent of probe wavelength and sample temperature. The absorption coefficients are similar at both probe wavelengths and the values at room temperature are about twice these at 10 K. We attribute the onset of the absorption to the ultra-fast formation of self-trapped excitons (STEs). The STEs recombine radiatively at a low temperature. At room temperature, we observe a cumulative effect. This demonstrates that, at 300 K, some of the STEs are converted to permanent colour centres, which we tentatively identify as neutral oxygen vacancies.

International audience; The study of the interaction between ultrashort laser pulses at relativistic intensities and solid matter has promoted, in recent years, a promising way of investigating ultrafast phenomena and has suggested the possibility of efficiently producing accelerated particle beams. As laser technology has been improving and increasing intensities and energies, it has become clear that the temporal contrast of a laser pulse is a key parameter in defining the conditions of interaction and the involved phenomena. In this paper we present the results that have been obtained in laser ion acceleration experiments from recent past to present days, using the most recent temporal cleaning methods. Different technologies have been adopted to improve the temporal contrast: the obtained effect on the proton acceleration is presented and discussed.

High-energy electrons can be produced in interactions of intense, ultra-short laser pulses with plasmas. Experiments conducted in the regime of moderate laser power (a few terawatts [TW]) are attracting increasing attention for their possibility of optimizing the acceleration process. Here we report the successful production of several-MeV electron bunches in interactions of femtosecond laser pulses from a 10 TW tabletop laser with supersonic gas-jets. The laser–plasma interaction and the obtained electron bunches have been characterized in detail, and conditions for stable and reproducible acceleration have been found. The accelerated electron bunches have been characterized by means of the measurement of the induced photo-activation of a gold sample via bremsstrahlung-generation of photons with suitable energy. The obtained result opens up a wide range of possible applications of the compact electron source for the concerns of nuclear physics studies. Some of them are briefly considered in this paper.

We used X-ray spectroscopy as a diagnostic tool for investigating the properties of laser-cluster interactions at the stage in which non-adiabatic cluster expansion takes place and a quasi-homogeneous plasma is produced. The experiment was carried out with a 10 TW, 65 fs Ti:Sa laser focused on CO2 cluster jets. The effect of different laser-pulse contrast ratios and cluster concentrations was investigated. The X-ray emission associated to the Rydberg transitions allowed us to retrieve, through the density and temperature of the emitting plasma, the time after the beginning of the interaction at which the emission occurred. The comparison of this value with the estimated time for the “homogeneous” plasma formation shows that the degree of adiabaticity depends on both the cluster concentration and the pulse contrast. Interferometric measurements support the X-ray data concerning the plasma electron density.

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8

We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity ($\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$), ultrahigh contrast ($g{10}^{10}$) laser pulse interacts with foils of thickness ranging from 0.08 to $105\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the $p$ component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.

Specular reflection is one of the most fundamental processes of optics. At moderate light intensities generated by conventional light sources this process is well understood. But at those capable of being produced by modern ultrahigh-intensity lasers, many new and potentially useful phenomena arise. When a pulse from such a laser hits an optically polished surface, it generates a dense plasma that itself acts as a mirror, known as a plasma mirror (PM). PMs do not just reflect the remainder of the incident beam, but can act as active optical elements. Using a set of three consecutive PMs in different regimes, we significantly improve the temporal contrast of femtosecond pulses, and demonstrate that high-order harmonics of the laser frequency can be generated through two distinct mechanisms. A better understanding of these processes should aid the development of laser-driven attosecond sources for use in fields from materials science to molecular biology.

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.

We report and discuss experimental results on the propagation of CPA pulses of moderately relativistic intensity in gas: they evidence the effects of the precursor pedestals of the main pulse. Details of great interest were observed for the first time with high quality femtosecond 90-degree interferometry. The interferometric data are also correlated with imaging and spectroscopy data of laser pulse transmitted through the gas. The most relevant physical features are confirmed by a numerical code which simulates the laser pulse propagation self-consistently with the ionization of the gas. We found that in this regime, the propagation of the intense femtosecond pulse is basically stable apart from very weak refractive effects. In order to allow propagation at fixed intensity along an optical path larger than the Rayleigh range, we performed a first successful attempt at producing hollow plasma channels able to guide the pulse.

By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 1018W/cm2 intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of highorder harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15% never achieved up to date in relativistic regimes.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we detail the new scheme that precompensates 30-fs, 10-TW laser beam aberrations in order to get the best laser focal spot on target. The adaptive optics loop consists of two four-wave lateral shearing interferometer as the wave front sensor. Wave front correction is achieved dielectric coated bimorph deformable mirror.

In this Letter, we demonstrate the instantaneous creation of a hot solid-density plasma generated by focusing an intense femtosecond, high temporal contrast laser on an ultrathin foil (100 nm) in the ${10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ intensity range. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate such a medium, enables the study of the dynamics of this plasma on the 100 fs time scale. The comparison of the transmission of two successive harmonics permits us to determine the electronic density and the temperature with accuracies better than 15%, never achieved up to this date in the regime of laser pulses at relativistic intensity.

International audience; Nonlinear elliptical polarization rotation is used to improve the contrast of femtosecond pulses by several orders of magnitude. Using nonlinear induced birefringence in air, we produced cleaned pulses with an energy of a few hundreds of microjoules. This technique presents several major advantages, such as convenience and stability of the setup. We investigated the phase profile required for obtaining high-energy pulses. No phase distortion is observed, and the spatial quality of the beam is preserved.

We investigate the harmonics generation from a pure dielectric target when submitted to laser intensities in the 1O'8W/crn2. We demonstrate the negative influence of the prepulses and ASE by addressing the direct comparison of the harmonic spectra with and without the introduction of a perfectly controlled plasma mirrorsystem. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free ofany plasma expansion. 1. INTRODUCTION It is now well known that, due to the non linear electronic response, the focalization of an intense laser pulse on anatomic vapour gives rises to an harmonics spectra consisting in narrow peaks separated by twice the frequencyof the laser driving field. For symmetry reasons, only odd harmonics can be produced. Such high harmonicgeneration makes avaible brilliant XUV sources, with pulse duration in the sub-picosecond regime. The emissionprocess is now well understood : under the laser field, an electron may be ejected into the atom continuum andreturn back to its initial ion when the electric field reverse its direction. It has then the possibility to recombine

We demonstrate the use of a plasma mirror to obtain 60-fs 10-TW laser pulses with a temporal contrast of 10(8) on a nanosecond time scale and 10(6) on a picosecond time scale, and we use these high-contrast pulses to generate high harmonics by nonlinear reflection on a plasma with a steep electronic density gradient. Well-collimated harmonics up to 20th order are observed for a laser intensity of approximately equal to 3 x 10(17) W/cm2, whereas no harmonics are obtained without the plasma mirror.

Summary form only given, as follows. The modeling of L-shell x-ray spectra from Kr clusters in laser plasmas is presented. Different types of nozzles create clusters composed of as many as (2-20) /spl times/ 10/sup 6/ Kr atoms, which are then irradiated by a high-intensity Ti:Sa 60-500 fs laser pulse. High-resolution (up to /spl lambda///spl Delta//spl lambda/ = 4000) L-spectra of Kr were obtained using X-ray Focusing Spectrometers with Spatial Resolution (FSSR). The spectrometers have been oriented to obtain spatial resolution of around 30-80 /spl mu/m in the direction perpendicular to laser propagation. A detailed time-dependent collisional-radiative model has been developed to diagnose the electron density, ionization balance, and electron distribution function of the plasmas both before and after the femtosecond pulse. The kinetic model and its application to laser plasmas from different nozzles are discussed in detail. Modeling was also performed on experimental spectra obtained from nanosecond laser interaction with Kr clusters. It is shown that the n=4 L-shell nanosecond laser plasma spectra can be adequately described by a single electron density and temperature, while similar spectra from femtosecond laser experiments require more sophisticated modeling with inclusion of hot electrons.

Spectra in the 7.10 to $8.60\mathrm{\AA{}}$ range from highly charged copper ions are observed from three different laser-produced plasmas (LPPs). The LPPs are formed by a 15-ns Nd:glass laser pulse (type I: ${E}_{\mathrm{pulse}}=1\char21{}8\mathrm{J},$ $\ensuremath{\lambda}=1.064\ensuremath{\mu}\mathrm{m}),$ a 1-ps Nd:glass laser pulse (type II: ${E}_{\mathrm{pulse}}=1\mathrm{J},$ $\ensuremath{\lambda}=1.055\ensuremath{\mu}\mathrm{m}),$ and a 60-fs Ti:sapphire laser pulse (type III: ${E}_{\mathrm{pulse}}=800\mathrm{mJ},$ $\ensuremath{\lambda}=790\mathrm{nm}).$ The spectra of high-$n$ $(nl~14)$ transitions in highly charged copper ions, ${\mathrm{Cu}}^{19+}$ to ${\mathrm{Cu}}^{21+},$ are recorded with a high energy resolution $(\ensuremath{\lambda}/\ensuremath{\Delta}\ensuremath{\lambda}=3000\char21{}8000)$ spectrometer using a spherically bent mica or quartz crystal. Collisional-radiative models are computed for the emission from each plasma. The sensitivity of the model spectra to opacity effects and to populations of superthermal electrons is studied. For the type I LPPs, opacity effects, treated with escape factors, are necessary to get the correct relative intensities of high-$n$ $(n=5,$ 6) Ne-like ${\mathrm{Cu}}^{19+}$ emission features. In the case of the type II LPPs, the contrast between the laser prepulse and the main pulse has been varied from low, ${I}_{\mathrm{main}}{/I}_{\mathrm{pp}}=7\ifmmode\times\else\texttimes\fi{}{10}^{4},$ to high, ${I}_{\mathrm{main}}{/I}_{\mathrm{pp}}=3.8\ifmmode\times\else\texttimes\fi{}{10}^{7}.$ For plasmas from low contrast shots, we find good agreement between the observed spectra and optically thin simulations with bulk electron temperatures ${T}_{\mathrm{bulk}}=0.4\mathrm{keV}$ and a small population of superthermal electrons ${(T}_{\mathrm{hot}}=5.0\mathrm{keV})$ that is ${f}_{\mathrm{hot}}l~{10}^{\ensuremath{-}5}$ of the bulk electron population. For high-contrast type II LPPs, we find higher densities and a combination of ${f}_{\mathrm{hot}}\ensuremath{\sim}{10}^{\ensuremath{-}5}$ and escape factors best describes the data. For the type III 60-fs LPPs, a population of superthermal electrons ${(T}_{\mathrm{hot}}\ensuremath{\approx}5\mathrm{keV})$ that is $\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ of the bulk electron population ${(T}_{\mathrm{bulk}}\ensuremath{\approx}0.2\mathrm{keV})$ is required to reproduce the observed spectra. The effect of both escape factors and hot electrons in the CR models is to increase the ionization balance and dramatically increase the number of strong lines for each ion considered. We have studied both opacity effects and hot-electron influence on high-$n$ transitions of highly charged Ne-, F-, and O-like ions.

The review of systematic investigations of X‐ray radiation properties of different clusters heated by short‐pulse (35–1100 fs) high‐intensive (1016– 1018 W/cm2) Ti:Sa laser radiation is presented. The cluster targets were formed by the adiabatic expansion in vacuum of an Kr or Ar gas jets produced by a pulsed valve with Laval or conical nozzles. The gas pressure is varied from 15 up to 100 bar. High spectrally (λ/δλ=4000–5000) and spatially (40–80 μm) resolved X‐Ray spectra near resonance lines (4‐2 transitions) of Ne‐like ions of Kr, H‐ and He‐like ions of Ar have been obtained and detailed spectroscopic analysis was consistent with a theoretical two‐temperature collisional‐radiative model of irradiated atomic clusters incorporating with an effects of highly energetic electrons. The role of laser prepulse for X‐ray intensity emission investigated in details. X‐ray spectra radiation from plasma with electron density more than 2×1022 cm−3 was observed. Big effect of fast electrons influence on the X‐ray emission of He‐like Ar and Ne‐like Kr spectra was demonstrated. Comparison of data obtained under various experimental conditions clearly showed that for increasing X‐ray output from plasma the most essential to increase size of clusters and has reasonable value of ps prepulse.

Summary form only given, as follows. Experimental and theoretical investigations of multicharged ions acceleration by an intense femtosecond laser radiation in wide (10/sup 17/-4-10/sup 18/W/cm/sup 2/) laser flux density range is presented. With the help of focusing spectrometers with spatial resolution (FSSR-1,2D) we have observed evidence of the emission of energetic He- and H-like ions in fluorine of more than 1 MeV produced via the optical field ionization (OFI) from solid target irradiated by intense I=(1-4)/spl middot/10/sup 18/ W/cm/sup 2/ (UHI-10 laser facility, 60 fs, /spl lambda/=800 nm), obliquely incident p-polarized pulse laser. The measured blue wing of He/sub /spl alpha//, He/sub /spl beta// and Ly/sub /spl alpha// lines of fluorine shows a Doppler-shifted spectrum due to the self-similar ion expansion dominated by superthermal electrons with the temperature T/sub h//spl sim/100-400 keV. Using a collisional particle-in-cell simulation, which incorporates the nonlocal-thermodynamic-equilibrium ionization including OFI, we obtained the plasma temperature, line shapes, hot electron spectra and maximal energy of accelerated ions, which agree well with those determined from experimental hot electrons and ions spectra. The red wing of ion spectra gives the temperature of bulk plasma electrons. By using simultaneously 3 FSSR spectrometers we also obtained X-ray spectra with 3D spatial distribution for He/sub /spl alpha//-K/sub /spl alpha// lines of Al, Ca and Cu targets. Such spectra clearly show that the size of emission zone of Ka radiation on the target surface in 2-5 times large, than the size of emission zone of Ka lines and their satellites. At the same time the size of all lines emission in the direction of laser-plasma expansion were equal. Interpretation of such an effect is presented.

Summary form only given. By means of space resolved X-ray spectroscopy we have investigated the generation of fast (up to MeV) ions for various laser installations with different flux densities (10/sup 12/-10/sup 14/ W/cm/sup 2/) and laser wavelengths (/spl lambda/=308 nm, 800 nm, 1.06 /spl mu/m, 10.6 /spl mu/m). The implementation of our spectroscopically determined data into the generally accepted scaling relation diagrams with the axes q/spl lambda//sup 2/ and E/sub ion/, where q is the laser flux density, /spl lambda/ the laser wavelength and E/sub ion/ the fast ion energy showed a large scatter by orders of magnitude. The simultaneous use of mass spectrometry showed reasonable agreement for both methods of measurements. Non-Maxwellian X-ray spectroscopy enabled us for the first time to investigate the generation of fast ions and hot electrons by means of space resolved spectroscopy in plasma areas, where the interaction with the laser beam takes place.

Summary form only given. The high-resolved X-ray spectra resulting from the interaction of argon gas clusters with short-pulse laser radiation was studied. The cluster target was formed by the adiabatic expansion in vacuum of an argon gas puff produced by a pulsed valve with a conical nozzle. The gas jet was characterized by interferometry. The density profile has a Gaussian shape with a 4 mm width (at 1/e), and the peak density is 4.6/spl times/10/sup 18/ cm/sup -3/ for a maximum gas backing pressure of 15 bars. The experiments were carried out using the UHI10 laser. It is a two-beam 10 Hz Ti:sapphire system (/spl lambda/=800 nm). In our particular experiments the main beam has a 7 TW peak power.